1. Field of the Invention
The present invention relates to an olefin polymerization catalyst comprising a metallocene compound having a substituted fluorenyl group having an electron-donating substituent on the benzo ring as one ligand, a cyclopentadienyl group as another ligand, and a hydrocarbon group, a silanediyl group, or a germanediyl group forming a bridge between the two ligands. The present invention also relates to a process for producing an olefin polymer of a high molecular weight at a high polymerization temperature of not lower than 120.degree. C. by use of the above catalyst.
2. Description of the Related Art
In polymerization of olefin, metallocene complex catalysts are known to be highly active which comprise as fundamental constitutional components a cyclopentadienyl derivative of a transition metal such as titanium, zirconium, and hafnium (Group 4 of Periodic Table) and an aluminoxane. Such catalysts are described by J. Boor ("Ziegler-Natta Catalyst and Polymerization", Academic Press, New York (1979)) and by H. Sinn and W. Kaminsky (Adv. Organomet. Chem. 1899 (1980)). These catalysts are shown to be highly active in olefin polymerization and to be capable of forming a stereoregular polymer. JP-A-1-503788 ("JP-A" herein means "Japanese Patent Laid-Open Publication") discloses a process for producing a polyethylene of high density or an ethylene/.alpha.-olefin copolymer of relatively high density by employing the aforementioned catalyst system comprising a metallocene compound and aluminoxane at a high pressure and a high temperature.
These catalysts, however, are not employed in commercial production, mainly because of the two disadvantages below. Firstly, aluminoxane, the cocatalyst, cannot readily be prepared with high reproducibility, whereby the catalyst and the resulting polymer cannot be produced with satisfactory reproducibility. Secondary, the expensive aluminoxane has to be used in a considerably high ratio to the transition metal compound, the main catalyst, in order to obtain high catalytic activity and stability of polymerization.
The above disadvantages are offset by an ionic metallocene catalyst. JP-A-3-207704 discloses an ionic metallocene compound prepared by reaction of a metallocene compound with an ionizing ionic compound. WO-92-01723 discloses a process for polymerization of .alpha.-olefin with a catalyst system prepared by reacting a halogenated metallocene compound with an organometallic compound and further bringing the resulting product into contact with an ionizing ionic compound, and describes advantages of such a catalyst system for olefin polymerization.
JP-A-5-320246 discloses high temperature polymerization with an ionic metallocene catalyst, where the polymerization catalyst is prepared from a known complex of dicyclopentadienylzirconium dichloride, dimethylanilinium tetrakis(pentafluorophenyl)borate, and triisobutylaluminum. However, ethylene/1-octene copolymers produced with this catalyst at high temperature have a low intrinsic viscosity, namely a low molecular weight. Therefore, the polymer produced with this catalyst is presumed to be insufficient in rigidity and strength for single use for plastics.
Generally, a polymer of a higher molecular weight is obtained at a lower polymerization temperature because of slower chain transfer reactions at a lower temperature. However, in polymerization at a temperature lower than the melting temperature of the polymer, the formed polymer deposits in the reaction vessel to retard agitation and to reduce the productivity. In solution polymerization where the polymerization is conducted at a temperature higher than the melting point of the polymer, the above disadvantages are offset, and the higher temperature decreases the viscosity of the polymerization solution to increase the agitation efficiency, thereby facilitating removal of polymerization heat and control of the reaction to produce a homogeneous polymer. In high-temperature high-pressure polymerization, the larger difference between the temperature of polymerization and the temperature of the feed of raw materials will increase the olefin conversion, and will improve profit. Accordingly, for high temperature polymerization, the metallocene catalyst is being investigated which is active under high temperature conditions.
After comprehensive investigation to solve the above problems, it was found by the inventors of the present invention that a metallocene compound having a specified substituent makes practicable the production of an olefin polymer of narrow composition distribution, narrow molecular-weight distribution, and high molecular weight even under high temperature conditions of not lower than 120.degree. C. The present invention has been completed on the basis of the above findings.